Purification and compression of ethylene



Dec. 9, 1947.4 E. HUIQTER 2,432,423

PURIFI'CATION AND COMPRESSION OF ETHYLENE Filed Octf. 4. 1943 Coolin or Heati g Coil :lease lre Pure [mylene Outlet Crude Mylene Inkl Presw'e Gaz/ qe Pure [Mylene yllebv Walernlet Q ATToRNfY particularly method of using ,pressure that Patented Dec. 9, 1947 uNiTeo STATE PURIFICATION AND COMPRESSION F v ETHYLENE i l Edward Hunter,Northwich,

England, assigner to Imperial Chemical Industries Limited, a corporation of Great Britain 4,194s,seriaiNo.5o4,95e inOctober 26,1942 i Application october In Great Brita 1 claim. (ci. 26o- 617) This invention relates to a new process for thel purification and compression of ethylene, in particular for the separation vated pressure from other gases.

Many -processes known for isolating an ethylene-rich gas from hydrocarbon and other gaseous mixtures containing small or large pr portions of ethylene, and ethylene-rich gases so obtained may contain 3095% of ethylene. `It is frequently necessary to purify this ethylenerich gas before it can be employed in further commercial processes, because some' of these processes require ethylene of a high degree of purity. This can be done to some extent by liqueiying the ethylene at an elevated pressure and a temperature below C., but such a process requires low temperature cooling and does not give a pure product because other gases are soluble in liquid ethylene.

One object of the present invention is to prof vide a new and improved process for the recovery of an' ethylene-rich gas from gases of intermediate or low ethylene content. Another object is to provide a process for the purification ofgases conutaining a major proportion of ethylene. Afur` ther object is to separate ethylene from hydro- Yet another yobject is for the thermal compression carbon andother gases. to provide a process of ethylene. i

It is known that ethyleneand water form a solid hydrate, probably hexahydrate, which can exist below 19 C. atan elevated pressure; the dissociation pressure of` this solid hydrate ranges from about 6 atmospheres at 0 C. to about 60 atmospheres at 17 C. We have now found that at sufficiently high pressures this hydrate is capable ofexisting at higher temperatures than hitherto thesubsequent decomposition of solid ethylene l hydrate. By raising the temperature of the hyof ethylene at an eledratethe ethylene is liberated at a pressure exceeding the pressure at which the hydrate `is initially formed. The temperature of the gas may be reduced `to the initial temperature if desired without dropping the pressure, bycooling the gas and allowing its volume to contract at constant pressure.

At the operating pressure during the absorption of ethylene and the formation of solid ethylene hydrate, the partial pressure of ethylene must exceed the dissociation pressure of ethylene hydrate atthe temperature employed. By dissociation pressureismeant the equilibrium pressure which is generated when ethylene hydrate coexistswith its saturated aqueous solution and thought, even up to 60 C. and we have devised a the formation and decomposition of this hydrate to give a new and improved process for the purification and/or compression of ethylene;

Accordingv to the present invention we contact ethylene or a gas containing it, with an aqueous medium preferably at a temperature between -5 and 60 vC.'and at such super-atmospheric at least part of the ethylene is converted into solid ethylene hydrate, and if desired separate the residual gas and/ or liquid rich in impurities, and thereafter raise the temperature and/or lower the pressure suiiiciently to `liberate ethylene from the ethylene hydrate.

By blowing off the residual gas or liquid, part or all of the impurities may be removed from the system and purified ethylene is obtained by the formation of ethylene hydrate` Naturally,`

ethylene hydrate at such because the solid does not 0 C., 20 atmospheresat 10 pressure for a vapour at vthe temperature employed. During the subsequent decomposition of ethylene must be raised above the decomposition temperature of ethylene hydrateat A ethylene i5 allowed to evolve, r thepressure must be lowered below the decomposition pressure of the temperature ofthe hydrate.` This decompositionis somewhat analogous to melting, but cannot properlybe described as lmelt into a liqsame composition but changes into a of which are of difuid of the solution anda vapour both `ferent compositions from that of the solid. `The lethylene hydrate at approximately` 6 atmospheres at .C., and '70 atmosf about `450 atmospheres dissociation pressure of, equilibrium is pheres at 20 C.; rising to at 40 C. and about 1000 `atmospheres at 60` C By partial pressure of ethylene in a gas mixture is meant the algebraic product of the total gas pressure and the mole thegasmixture. The above dissociation. pressures represent the minimum ethylene partial pressures requiredat these temperatures to cause the lower the mole fraction of ethylene .in the gas the higher will be the the formation of the hydrate. In order to absorb the ethylene at reasonable pressures,we prefer touse temperatures as low as can conveniently be obtained.` Thus, at 20"` C. it is sufficient to 'compress the incoming` gas so that the partial pressure of ethylene in that gas exceeds '10 atmospheres; whereas at 40` C. and

` C. ethylene partial pressuresvexceeding 450 atmospheres and l1000 atmospheres respectively are required. At thesame time, it is not always hydrate y fraction of ethylene in Y above factors.

convenient to work below atmospheric temperav tures and therefore We prefer to" operate at 30 C. Any ethylene cess ofthe above ing force for the solid ethylene hydrate. use any total pressure at and batch may be 1/g-20 hours, depending on all the eration of gas at 250 temperature above 31 position temperature of ethylene pressure of the to about three quarters of it.

During the absorption of ethylene mosphereaan appreciable weight of ethylene can be converted into process.

One method of eiecting this heat change durg the absorption and ethylene thermal contact of these two .stages of the process. In this Way the heat which must be imparted to the ethylene puried ethylene 1s evolved are of the incoming gas. One suitable apparatus in which this form of the process can be carried out ethylene.

A more detailed understanding of the apparatus will be obtained from the accompanying drawings `which are cross-sectional, diagrammatic views of suitable apparatus.

In Figure 1 a pressure resisting receptacle is provided with Water inlet, crude apparatus iskfrst lled with Water, crude ethylene unabsorbed gases exhausted through the release valve. When a suitable vessel heated by the relatively pure ethylene discharged.

-increase of total pressure the liquid becomes rich in nitrogen.

Figure 2 illustrates a nest of similar receptacles suitably insulated, whereby the exothermic heat formed by crystallization in one receptacle is utilized to heat the hydrate in the other above the hydrates dissociation temperature. By this means no external heat is required to conduct the process.

Figure 3 illustrates an apparatus wherein a This invention also provides a new type 'of heat compressor for ethylene which may be illustrated Starting with ethylene at a partial,

as follows. pressure of '70 atmospheres and 20 C., we pass it into water, which is kept below 20 C. by external cooling, until a substantial amount' of ethylene hydrate is formed. We then raise the temperature of the ethylene hydrate to 60 vessel closed, and while maintaining this temperature by applying heat we allow 'the hydrate to decompose and liberate the ethylene at apres- The ethylene at a. pressure of 1000 atmospheres is also at a higher temperature than initially, Cooling the ethylene at constant volume reduces somewhat the net amount of compression effected by the process; alternately, the ethylene may be cooled at constant pressure as it issues, This heat compressor can also be operated at lower or higher pressures and temperatures, provided that the temperature of decomposition of the hydrate is higher than the temperature of The purification of ethylene may be illustrated by the following description. When any gas is compressed in the presence of water the gas dissolves up to an amount proportional to the partial pressure of that gas. of ethylene, the-concentration of ethylene in solution at a. given temperature below 60 C. is limited by the formation of solid ethylene hydrate whenthe pressure is sufficient; on the admission of further ethylene the equilibrium pressure does not rise because the ethylene is converted into solid ethylene hydrate. and the solution has only a low ethylene content. In practice, the'actual partial pressure exceeds the equilibrium pressure n by an amount representing the difference between j C. with the formation. f

In the particular case g such gases are principally acetylene,carbon the rate of input of ethylene and the rate of crystallisation of hydrate. The compression of ethylene in the presence of water which is kept cold gives at rst a rise of pressure while the water phase is being enriched with ethylene, and then I a fairly steady pressure despite further compression, while ethylene hydrate is being deposited and the composition of the solution is /remaining unchanged. For example, if we compress a gaseous mixture containing ethylene and nitrogen in the presence of water at a xed temperature,

these two gases at first dissolve in quantities which r correspond with their partial pressures. As the total pressure of gas is increased the concentration of ethylene in the water reaches its maximum figure at that temperature and a further does not increase the ethylene partial pressure. ethylene extracted from the incoming gas is increased disproportionately to the amount which would dissolvev if there were no separation of ethylene hydrate, and the gaseous mixture above A sufficiently pure ethylene may usually be obtained by blowing-ofi 'or otherwise displacing the gas mixture Thus, the amount of.

vrich in nitrogen ethylene obtained coming gas inally present is converted to solid ethylene hynated.V The separation from Vby the following examples.

from the top of the vessel and. subsequently raising the temperature` or lowering the pressure of the liquid plus solid phase sufficiently to effect dissociation of the solid phase and thereby liberate purified ethylene. Thisl effects a large measure of purification, but the is still slightly contaminated with nitrogen due to the small quantity of nitrogen dissolved in the liquid phase present. An even more complete purification may be effected by not only purging the nitrogen-rich gaseous phase, but also either removing the liquid phase leaving pure ethylene hydrate from which Vpure ethylene is recovered, or utilising so much inthat substantially all the water origdrate.

lIt is convenient to observe the progress of the purification by the composition of the gas mixture above the liquid. When solid hydratestarts to form, the ethylene content of the supernatant gas falls below that of the incoming gas.` A further method is by observing the rate `o`f cooling required to prevent the temperature of the liquid from rising, because crystallisation of the hydrate is accompanied by evolution of heat.

The invention may be employed for the separation of a purified ethylene from gases containing ethylene and one or more of the following gases: other hydrocarbons, hydrogen, nitrogen, oxygen, carbon monoxide, carbon dioxide, and all other gases with which ethylene is normally contamiv most`of these gases is virtually complete, but it is incomplete from those gases which also form solid hydrates within this temperature range, when large amounts of such gases are ptesent in the incominggas; dloxide and methane. The process has the advantage that the purified ethylene may be recovered at substantially the same pressure as the incoming crude ethylene, or even higher.

The invention may Vconveniently be operated as a batch process, 4in which each reaction vessel may be used first as an absorber and crystalliser, and later as a decomposer. It may also be operated as a semi-continuous process in which a series of coupled up alternately to provide in effect a continuous purifica-tion and/or compression of ethylene.` It may also be operated by continuous absorption of ethylene and crystallisation of the hydrate, in a, vessel to which water and compressed4 ethylene are fed continuously, with batch or continuous transfer of the hydrate therefrom to a continuous decomposer.

The invention is illustrated but not restricted Example 1 f The reaction vessel employed in this example is a stirred pressure vessel fitted with a release valve set to operate at 100 atmospheres. The vessel contains 400 parts of water by weight, and there isfa small vgas space above thesurface of the water; the incoming gas contains by vol-- ume of ethylene and 10%i by `volume of other hydrocarbons, and it is compressed into the relaction vessel. The iirst amounts of gas to be compressed into Vthe pressure vessel partly dissolve and the pressure rises rapidly to 80 atmospheres and then to atmospheres. ThereafterV there is a continuous ypurge of impure gasthrough the release valve. This impure gas contains 75% by volume of eth'yle'ne, the remainder being the other hydrocarbons introduced with the impure generally prefer to use gases containing at least gas. 90 parts by weight of the incoming gas are 60% of ethylene. compressed into the reaction vessel, which is WhatIclaim is:

contains nearly all the impurities remaining in ing a vessel with water leaving a small vapor tion vessel raised to '30-40 C; and 50 parts by ter under conditions at which ethylene hydrate Weight of gas are thereby evolved at 300 atmosforms, while discharging the unabsorbed gases pheres pressure. This gas' contains 9899% by 15 through the vapor space from the vessel, after the volume of ethylene. formation of ethylene hydrate adding more wa- Eample 2 ter to the vesselto ll the vapor Spare, thereby A Cooled agitated P1 eseure VeSSe1 With eonnee' sel and thereafter recovering the ethylene by subn tions at the upper and lower ends, is half 1 1d 20 jeenng the ethylene hydrate to conditions than with cold water. A compressed gas containing dissociaweitq 70 volume of ethylene and 30% volume nitro- EDWARD HUNTER, gen is bubbled into the water. When the pressure has risen to 500 atmospheres the gas feed is l y REFERENCES CITED The following references are of record in the le of this patent:

from the bottom of the vessel, and the solid re- UNITED STATES PATENTS maining in the vessel -is heated to generate eth- Number Name Date dem v a0 2,271,214 Welty Jr Jen. 27, 1942 Example 3 2,270,016 Benesh Jan, 13, 1942 A gas containing 99 volume ethylene, 1 vol- 2,363,529 Hu'tChinSOn NOV- 28, 1944 ume hydrogen, is compressed and bubbled into 2,356,407 HutChinSOn -A- Aug. 22, 1944 water which is maintained at C. in a pres- 2,263,363 MeIlShih NOV. 13, 1941 sure vessel. The pressure in the vessel rises to 2,375,559 Hutchinson et al, May 3, 1945 350 atmospheres and at that pressure impure gas 2,375,560 Hutchinson et al. May 8, 1945 is allowed to escape from the top of the vesse1 2,399,723 Crowther May 7, 1946 while the solid hydrate forms in the vessel.- The OTHER REFERENCES ylene containing 99.7% ethylene is evolved at Hammersehmidt Jour- 1nd- Eng Chem V01- 350 atmospheres pressure. 26, 851-5 (1934) P 0- L- 'The invention may be employed for purjfyjng Villard, Annales de Chemie et de Physique ser. crude or fairly pure ethylene and for separating 7 v01-11, 36E-371 (1397)260-675 ethylene from an ethylene-rich gas. The former Beaton et a1" American Gas Jour-v June 1937, gas will generally contain over oi ethylene 45 17-21 9114132- (1311011059942 111'260676-1 by volume, but the latter may contain even as Beaton et al" "Gasf June 19401 28-30- (P110- little as 10% of ethylene by volume although'we 9059912111 26o-6769 

